Switching Mode Power Supplies (SMPS) are used in a variety of electronic devices including laptop computers, cellular phones, personal digital assistants, video games, video cameras, etc. They may convert a dc signal at one voltage level to a dc signal at a different voltage level (this is a dc-dc converter), an Alternating Current (ac) signal to a dc signal (this is a an ac-dc converter), a dc signal to an ac signal (this is a dc-ac converter), or an ac signal to an ac signal (this is an ac-ac converter). Generally, switching mode power supplies transfer energy from an input node to an output node by means of a switch, an inductor, and control and feedback circuitry. One type of switching mode power supply topology is a Buck converter in which the voltage appearing at the output node is stepped down from the voltage appearing at the input node. A Buck converter may include a high side Field Effect Transistor (FET) and a low side FET, where the drain of the high side FET is coupled for receiving an input signal, the source of the high side FET is commonly connected to the drain of a low side FET and to a terminal of an inductor, and the source of the low side FET is connected to ground. The gates of the high and low side FETs are coupled for receiving corresponding control signals. The other terminal of the inductor is connected to a load.
To optimize the efficiency of a converter such as, for example, a buck converter, it is desirable to prevent a negative current from flowing in the low side FET because the negative current increases conduction losses from the FET which results in an increased power loss. Thus, a Buck converter is typically operated in a discontinuous operating mode to reduce conduction losses. In this operating mode the low side FET is turned off when the inductor current reaches zero. A drawback with turning off the low side FET is that if it is turned off before the inductor current reaches zero, current continues to flow through the body diode of the low side FET which increases power losses. Conversely, if the low side FET is turned off too late, a negative inductor current causes current to flow through the body diode of the high side FET that is off, which causes the converter circuit to ring thereby producing ElectroMagnetic Interference (EMI).
Accordingly, it would be advantageous to have a method and circuit for adaptively adjusting the turn off of the low side FET in a switching mode power supply. It would be of further advantage for the circuit and method to be cost efficient to implement.